As a rule, a major number of semiconductor devices are manufactured simultaneously on monocrystalline silicon discs, so-called wafers. In so doing, the circuits of the semiconductor devices are usually structured on the wafer in a plurality of process steps. The semiconductor devices are individualized by the wafer being sawn, and are housed in plastics material. Frequently, the semiconductor devices are examined by appropriate test systems or test stations for their operability even prior to the individualization.
Thus, the semiconductor devices, such as integrated computing circuits, semiconductor memory devices, e.g., functional memory devices and table memory devices, etc., can be subjected to extensive tests at a plurality of test stations in the semi-finished and/or in the finished state. For testing the semiconductor devices, an appropriate test device may be provided at the respective test station, the test device generating the test signals required for the testing of the semiconductor devices.
The present invention especially serves to be used during the testing of the operability of semiconductor devices with appropriate test systems or test devices. To electrically connect the semiconductor device to be tested in a test station with the test system, a specific contact device, a semiconductor device test card a so-called “probe card” is commonly used. At the semiconductor device test card there are provided needle-shaped pins contacting the corresponding contact pads of the semiconductor devices to be tested.
By means of the probe card, at a test station, the signals required for testing semiconductor devices that are still positioned on the wafer can be generated by the test device that is connected with the probe card and can be introduced into the respective contact pads of the semiconductor devices by means of the contact needles provided at the probe card. The signals output by the semiconductor device at corresponding contact pads in reaction to the test signals input are in turn tapped by needle-shaped pins of the probe card and are, for instance, transmitted to the test device by a signal line connecting the probe card with the test device, where an evaluation of the corresponding signals may take place. After the sawing apart of the wafer or the individualizing of the semiconductor devices positioned on the wafer, respectively, these may then be loaded in a corresponding outer package, so-called carriers, and be transported to further test stations.
At a further test station, the carriers with the semiconductor devices to be tested may be inserted into corresponding adapters or sockets that are connected with a test device, and subsequently the semiconductor device positioned in the carrier may be subject to further test methods. For testing the semiconductor devices positioned in the carriers, the test signals output by the test device are transmitted, via an adapter and corresponding pins of the carrier, to the contact pads of the semiconductor device to be tested. The signals output by the semiconductor devices at the corresponding contact pads in reaction to the test signals input are tapped by the carrier pins and transmitted to the test device via the adapter (and a signal line connecting the adapter with the test device), where an evaluation of the signals may take place.
To achieve, with the test methods, a high accuracy of the signals introduced into the semiconductor device to be tested or of the signals measured, respectively, the respective test device has to be subject to a calibration or set-up process prior to the actual test method. To this end, the respective test device may, via a signal line, output a corresponding calibration signal to a probe card contacting the semiconductor device to be tested or the carrier carrying the semiconductor device, respectively. Subsequently, the test device may measure and evaluate the reflection signal caused by the calibration signal.
A further condition for the exact performance of a test method is the reliable contacting of the semiconductor device to be tested with the test system or with the test device, respectively. To this end, a good electrical connection between the contact needles of the test system and the contact pads of the semiconductor device to be tested has to be guaranteed.
At the beginning of the process for testing the operability of the semiconductor device, the contact needles or the probe card of the test device, respectively, are/is usually positioned above the semiconductor device to be tested such that the respectively desired pins or contact needles of the test device each contact the desired connections or contact pads of the semiconductor device to be tested.
After the testing of the semiconductor devices at wafer level, the impressions left by the contact needles on the contact pads of the chips are examined with respect to size and position. This is necessary to supervise the process of probing and to correct deviations in the position of the contact needles vis-à-vis the contact pads. If the impression of the contact needles exceeds the edge of the contact pad, no electrical connection between the contact needle and the contact pad has been established, and damage of the passivation surrounding the contact pad may occur, which constitutes a potential risk of reliability during later use.
Due to manufacturing tolerances and deviations in the position of the semiconductor devices to be tested on the wafer or in the carriers, respectively, there may also occur defective or missing contacting between the contact needles and the contact pads. Therefore, an optical inspection of the contact faces or contact pads at semiconductor devices is performed after the contacting of the contact pads by the contact needles to determine the actual position of the contact needles on the contact pads of the semiconductor device.
On the surface of the contact pads, the contact needles leave a trace in the form of an impression that occurs optically as a shading. By the optical inspection of the contact pads, the impression or the position of the contact needle during the contacting on the contact pad can be detected by means of this shading. If the detected position of the contact needle does not lie on the contact pad or not in the desired region on the contact pad, respectively, the position of the contact needles or of the probe card, respectively, of the test device may, in a subsequent adjustment step, be corrected correspondingly vis-à-vis the contact pad of the semiconductor device to be tested.
By means of the hitherto used prior art test systems and tools for the optical inspection it is, however, only possible to analyze impressions of contact needles on contact pads with relatively simple contact pad geometries (e.g., in the form of a rectangle, a rectangle with beveled corners or round contact pads). Any modification of the contact pad geometry and any modification of the surface structure of the contact pads require an adaptation of the software algorithms of the image recognition.
Furthermore, the impressions of the contact needles on the contact pads of semiconductor devices with modified chip design such as it is, for instance, used with current semiconductor memory chips with the highest treatises, cannot be judged meaningfully with the existing methods and test systems. Due to modifications of the layers positioned below the contact pad, modifications of its surface may occur. Consequently, by means of conventional image recognition systems, the center of the contact pad which appears somewhat darker vis-à-vis the surrounding region cannot be differentiated from the impression of the contact needle, this resulting in that the position of the impression of the contact needle is determined incorrectly. In such cases the size of the impression of the contact needles on the contact pad cannot be interpreted correctly since, e.g., a too large region is interpreted as an impression of the contact needle. Moreover, the distance to the edge of the contact pad may be determined incorrectly.
With known methods, for every new contact pad design that could not be analyzed with existing algorithms, the analysis system hitherto had to be programmed with a new algorithm. The procedure of the new programming of the analysis system is, however, time consuming and cost intensive. Moreover, all the hitherto used algorithms are based on the principle of identifying the impression of the contact tips on the contact pads of the semiconductor device by means of the modified contrast of the optical appearance, which is only possible with difficulty in the case of a poor contrast and leads to unreliable results.
For these and other reasons, there is a need for the present invention.